2 year old border collie. Such a loving, gentle soul, a really good natured dog. Last night we were out walking on a quiet country road, somewhere we walk very regularly, he knows the route well. A white car was parked at the side of the road and as we got close, my dog went all stiff and started growling. His eyes were fixed on the car. This is the first time I've heard him growl in his whole life! And then he started barking between growls. As we moved past the car he wouldn't take his eyes off it, and he continued to do a low growl. As soon as we were a few metres past it he went back to normal. We see cars all the time both parked and moving and he's never had an issue! He's never been a barker either so this has worried me slightly, as to why he would do this out of the blue.
Any ideas?